I use a 9" 5k stall for motor only. My nitrous converter is around 4k stall.
Both done by Atlantic Coast Converters in Conway, SC. Couldn't be happier w/ all the converters they've built for me over the past 20 years.

CHIP
'70 hemicuda, 575" Hemi, 727, Dana 60
'69 road runner, 440-6, 18 spline 4 speed, Dana 60
'71 Demon, 340, low gear 904, 8.75
'73 Chrysler New Yorker, 440, 727, 8.75
'90 Chevy 454SS Silverado, 476" BBC, TH400, 14 bolt
'06 GMC 2500HD LBZ Duramax